I believe that in this specific case Spotify makes a very compelling argument. App publishers that do not directly compete with Apple have a level playing field although the Apple tax certainly hurts. But Spotify directly competes with Apple, and while Apple does not have to pay this tax as they can (probably) use their own billing services cheaper, Spotify couldn't do the same even if they implemented their own bill…
You make money off your app? Because apple made it discoverable in the app store! You make money off your app, pay a share back for os/api r&d!